The discussion centers on the rotational symmetry of the ammonia (NH3) molecule, specifically addressing the number of rotation axes it possesses according to different point group classifications (C3v and C2v). Participants explore the implications of these classifications and the nature of symmetry elements in molecular geometry.
Participants express disagreement regarding the number of rotation axes for NH3, with some asserting the presence of a C2 axis while others maintain that NH3 only has a C3 axis. The discussion remains unresolved, with multiple competing views presented.
Participants reference different point groups and symmetry elements, leading to potential confusion regarding the definitions and implications of these classifications. The discussion also highlights the importance of distinguishing between planar and non-planar molecular geometries in the context of symmetry.
ampakine said:According to the character table for the C3v point group there are 2 C3 rotation axes along the z axis. How are there 2 of them? There is only 1 listed in the C2v point group table which makes sense but I can't see how a molecule like NH3 has more than one rotation axis along the z axis.
DrDu said:There is only one axis, but two (or in reality 3) rotations (=symmetry elements): rotations by 0, 120 and 240 degrees.
DrDu said:There is only one axis, but two (or in reality 3) rotations (=symmetry elements): rotations by 0, 120 and 240 degrees.
Silex7 said:who said that ammonia has 2 rotations axes? it has only one rotation axis which is C2 axis,
check here :
http://www.chemeddl.org/labs/avisualdatabase/index.php
and look for ammonia, and see symmetry properties.
How can a 180 degree rotation of a trigonal pyramidal molecule be a symmetry element?Silex7 said:who said that ammonia has 2 rotations axes? it has only one rotation axis which is C2 axis,
ampakine said:How can a 180 degree rotation of a trigonal pyramidal molecule be a symmetry element?
Silex7 said:look if you have a type of AB3 molecules with similar 3 atoms like NH3 then you have one rotation improper axis which is C2 this axis is passing through each H atom hence you have only one C2 axis since the 3 atoms are the same, but how does this axis process 180 symmetry?..this needs a bit visualization from you..
if you have a planar NH3 (that is a molecule drawn on planar paper with no 3D dimensions) and keeps in your mind the image of the C2 passing through one of the H atom, if you turn the paper to 180 degree (turn the paper to other side) you will have the left H atom in the right position and versa. and this is also with other atoms.
Notice also that NH3 has also C3 axis but this once passes through Nitrogen atom perpendicular to it (axis goes from the paper to out), that is if you rotate the molecules around the inner axis by 120 degree(that is the angel between 2 H atoms) you will actually have the same H positions which is another symmetry.
got it?
SpectraCat said:Except that NH3 is not planar .. you are describing D3h symmetry .. NH3 has C3v symmetry, and the OP is correct that it has no C2 axes.
Silex7 said:Yes the trigonal pyramidal structure has only the C3 axis but there is no C2 axis, i mentioned the trigonal planar structure in my previous discussion.
http://www.chemeddl.org/labs/avisualdatabase/index.php?molecule=1&sort=name&kind=all&molecule2=-1&sort2=name
Silex7 said:who said that ammonia has 2 rotations axes? it has only one rotation axis which is C2 axis,
check here :
http://www.chemeddl.org/labs/avisualdatabase/index.php
and look for ammonia, and see symmetry properties.
ampakine said:What about BrF5 then. It belongs to the C4v point group and according to here:
http://www.webqc.org/symmetrypointgroup-c4v.html
it has 2 C4 rotations and a C2 rotation. SpectralCats explanation works better here because if a 270 degree rotation can be considered a C4 rotation then why couldn't a 180 degree rotation?